1. Field of Invention
The present invention relates to an Fe-Ni alloy used for a shadow mask and a method for producing a shadow mask. More particularly, the present invention relates to an Fe-Ni alloy used for the shadow mask of Braun tube, which is produced by fine photo-etching of an Fe-Ni alloy sheet for piercing, annealing and bending by means of warm pressing. The present invention particularly improves the softening property of an Fe-Ni alloy sheet, when it is annealed at a temperature of 800xc2x0 C. or more before the press-forming.
2. Description of Related Art
FIG. 1 is a cross-sectional view of the known shadow-mask type color cathode-ray tube. A fluorescent coating 2 is applied on the panel 1 and emits three primary colors, i.e., red, green and blue. An electron gun 4 is provided in the neck of the cathode-ray tube and emits electron beams 3. The electron beams 3 are deflected and scanned by the deflecting yoke 5. Reference numerals 6 and 7 denote the shadow mask and the magnetic shield, respectively.
Referring to FIG. 2, an enlarged partial view of the shadow mask 6 is shown. The electron beams from the red, green and blue electron guns 4a, 4b and 4c pass through one of the apertures 9 of the shadow mask 6 and then energizes the fluorescent dots 10.
Heretofore, aluminum-killed steels have been used for the shadow mask of a Braun tube. An Fe-Ni alloy referred to as a 36 alloy or Invar is used for a high-grade device from the point of view of color purity, because its thermal expansion is low and its gas emission upon exposure to high vacuum or electron impingement is minimal.
Photo-etching is generally used for piercing the apertures 9 through the shadow mask 6, from the viewpoint of dimension accuracy. A warm press is used for bending the Fe-Ni alloy into a curved shape of the panel 1. The Fe-Ni alloy must exhibit two properties. One property is etching property such that apertures 9 are pierced by etching with the pitch and area size exactly as designed. The other property is press formability such that the sheet is bent exactly as designed.
In order to meet the above mentioned etching property, there are various proposals to decrease the non-metallic inclusions in the Fe-Ni alloy. There is a proposal in Japanese Unexamined Patent Publication (kokai) No. 7-48651 to decrease the oxygen content and the amount of oxide inclusions so as to improve the press formability.
In order to decrease the amount of oxygen, expensive refining method, for example a two-stage refining such as the preliminary, vacuum melting or ladle refiing and the secondary re-melting by vacum arc melting or electroslag melting, becomes necessary. Such method may be not be feasible industrially.
There is a proposal in Japanese Unexamined Patent Publication (kokai) No. 9-324244 that the size of non-metallic inclusions in an Fe-Ni alloy is controlled by means of accelerating the cooling rate in a temperature range of from 1100xc2x0 C. to 700xc2x0 C., when it is hot-rolled from the hot-rolling temperature-range prior to the subsequent cold-rolling step. As a result, the softening temperature is lowered to a level lower than that attained heretofore. In order to implement such controlled cooling, a cooling equipment and electric power for the cooling equipment are necessary. In addition, the present inventors discovered that the etching property of the so-controlled cooled Fe-Ni alloy sheet is not improved in a case the Al content of such sheet is high.
It is an object of the present invention to provide an Fe-Ni alloy used for a shadow mask, having improved etching property and press formability.
It is another object of the present invention to provide a shadow mask made of an Fe-Ni alloy having improved etching property and press formability, which is annealed at a particular range of temperature.
It is a further object of the present invention to provide a method for producing an Fe-Ni alloy used for a shadow mask, in which the previously proposed methods, i.e., the special refining and melting method and the accelerated cooling, are unnecessary.
The present inventors considered various ways to improve the etching property and discovered that the etching property is further improved with higher N content of the Fe-Ni alloy. The present inventors quantitatively analyzed the influence of Al and Mn upon the etching property. As a result of these findings and analysis, the following Fe-Ni alloy for a shadow mask is provided.
In accordance with an object of the present invention, there is provided an Fe-Ni alloy used for a shadow mask, consisting of, by weight percentage, from 34 to 38% of Ni, up to 0.5% of Mn, from 0 to 0.02% of soluble Al, from 0.0030 to 0.0100% of N, the balance being Fe and unavoidable impurities, in which the smaller of the first value, which is the content of said soluble Al content divided by 27, or the second value, which is the nitrogen content divided by 14, is not more than 0.00015, and in which, when the first value and the second value are the same, the content of soluble Al is from 0 to 0.01%.
There is also provided a shadow mask consisting of an Fe-Ni alloy sheet bent by a press forming to shape conforming to the shape of the panel of a Braun tube, and having apertures formed by photoetching through which an electron beam passes, said alloy consisting of, by weight percentage, from 34 to 38% of Ni, up to 0.5% of Mn, from 0 to 0.02% of soluble Al, from 0.0030 to 0.0100% of N, the balance being Fe and unavoidable impurities, in which alloy the smaller of the first value, which is the content of said soluble Al content divided by 27, or the second value, which is the nitrogen content divided by 14, is not more than 0.00015, and in which, when the first value and the second value are the same, the content of soluble Al is from 0 to 0.01%.
There is further provided a method for producing an Fe-Ni alloy used for a shadow mask comprising the steps of:
preparing a sheet of an Fe-Ni alloy consisting of, by weight percentage, from 34 to 38% of Ni, up to 0.5% of Mn, not more than 0.02% of soluble Al, from 0.0030 to 0.0100% of N, the balance being Fe and unavoidable impurities, in which alloy the smaller of the first value, which is the content of said soluble Al content divided by 27, or the second value, which is the nitrogen content divided by 14, is not more than 0.00015, and in which, when the first value and the second value was the same, the content of soluble Al is from 0 to 0.01%;
photo-etching said sheet;
annealing said sheet at a temperature of 800xc2x0 C. or more; and,
bending said annealed sheet to the form of the shadow mask.
The present invention is described hereinafter in detail.
The soluble Al herein is the solute Al dissolved in the matrix of an Fe-Ni alloy and is distinguished from the insoluble Al which is present as inclusion such as oxide inclusions. The content of soluble Al is analyzed by: dissolving Fe-Ni alloy in hydrochloric acid or a mixture hydrochloric acid and nitric acid, and subjecting the filtrated liquid to analysis by the ion-coupled plasma activated Auger electron spectroscopy (ICP-AES method). The insoluble Al can be analyzed by dissolving the filtration residue by aqueous sodium peroxide solution and then to the ICP-AES method.
The etching property is influenced by the soluble Al. In addition, the annealing softening property is succeptible to change by the lower content of the soluble Al content and the N content.
The stoichiometric amounts determining the amount of aluminum nitride (AlN) are the first value, which is the soluble Al content divided by the atomic number of Al, i.e., 27, and the second value, which is the nitrogen content divided by the atomic number of N, i.e., 14. When the first value is greater than the second value, the soluble Al may remain in the matrix of the Fe-Ni alloy and the amount of AlN is determined by the second value. When the first value is smaller than the second value, the unfixed N may remain in the matrix and the amount of AlN is determined by the first value. The annealing softening property is influenced by the smaller of the first and second values, which is preferably 0.0001% or less. In other words, these values, which determine the amount of AlN, exert influence upon the annealing softening property.
In a case, where the first and second values are equal, neither soluble Al nor unfixed N may remain in the matrix of the Fe-Ni alloy. When the first and second values are equal, the formation of AlN should be suppressed to an extremely low level. The soluble aluminum content is preferably from 0 to 0.01% by weight.